Structural Distortion of the TpCo-L Fragment (Tp = tris(pyrazolyl)borate). Analysis by X-ray Diffraction and Density Functional Theory

The crystal structures of coordinatively unsaturated [(Tp Np Co) 2 (μ-N 2 )] and Tp Np Co(CO) show “bent” molecules, in which the fourth ligand (N 2, CO) is bent away from the pseudo-threefold axis of the TpCo-moiety by 27−38°. Magnesium reduction of Tp t -Bu,Me Co(CO) yielded [Tp t -Bu,Me Co(μ-CO)] 2 Mg(THF) 4 which was also structurally characterized; the reduced carbonyl is “linear”. Extended Hückel theory (EHT) and density functional theory (DFT) have been used to analyze the electronic structures and structural preferences of the TpCo-L fragment with L = CO (Co I, d 8 ), COLi (Co 0, d 9 ), and I (Co II, d 7 ). The actual and theoretical structure determinations were in good agreement. Based on these results we suggest that d 8 TpCo-L complexes and, by analogy, isoelectronic CpM-L complexes assume “bent” structures.